# Pressure/temp vs trajctory

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by sierra22, Jul 23, 2004.

1. ### sierra22Well-Known Member

Messages:
126
Joined:
Dec 12, 2002
Could use a bit of help with my problem, as I'm somewhat a math-lectic.

PROBLEM: Correcting trajectory for measured barometric pressure and temperature to score first round hits without sighters.
Trajectories are to my understanding calculated according to a standard atmosphere. Change this and the bullet will behave differently and the calculated BC will change.

SO FAR: If barometric correction X temperature correction X original BC = new BC
and
Barometric corr = Standard pressure : measured pressure,
Temp corr = measured Rankin temp : standard Rankin temp

...then I can calculate the deviations (not correctly, but better than not doing it) by working out a new dropchart for the new BC.

I WANT: To find correction factors where I can correct directly from a trajectory chart without going the long way around and making a new one.

Any takers ?

[ 07-23-2004: Message edited by: sierra22 ]

2. ### Richard338Well-Known Member

Messages:
126
Joined:
Aug 22, 2003
Most programs allow you to input environmental variables and generate a new trajectory without changing the BC. The program just changes the air density and speed of sound accordingly.
Try JBM's ballistic site.

3. ### MikecrWell-Known Member

Messages:
4,264
Joined:
Aug 10, 2003
Your looking for a way to apply a percentage to your bullets path for a difference in air density?
Like if today's temp is the same, but a front is coming, and today's pressure is 1" of mercury lower. How will that effect my drop today?
Dude, that would be a seriously ugly formula to carry in the field.
However, various common scenarios could be run on a program, and then a list of rough corrections could be made & carried along.

[ 07-23-2004: Message edited by: Mikecr ]

4. ### sierra22Well-Known Member

Messages:
126
Joined:
Dec 12, 2002
Yes, you all understand the intention with my question correctly. Complicated yes, but satisfactory fun if I can get it working.
True, sighters or several charts will take care of (parts of) the problem.

However, I know hard target interdiction operators correct their MOA on a "fill-in-the-blanks" formula using constants calculated for the various ranges, AND hitting their targets.

I have the data for the Raufoss .50 MP ammunition and I'm familiar with using the form and the calculator, but I don't understand the process of finding the variables that would be valid for other calibers than the .50.

To illustrate:
In the past I calculated a chart for the 600m range I sometimes use, but it was never "quite right" even though very, close. In winter it wasn't all that close.
On a mild winter day I'd get 52 instead of 48 clicks (1/3 MOA scope) for elevation. This I accredited to reduced powder burn rate.
For windage I could get about 41 instead of 37 clicks for windage (almost constant wind from the sea), which I accredited to poor wind calling.
Of course these errors would get larger with range, but 600m is all I have collected data for.
However, when I was taught some neat tricks and borrowed a wind/barometer/termometer, calculated the new BC it all got quite a bit closer.

A calculation could look like:
(stand. pressure: measured press.)x(m. temperature:stand. temp.)x manufacturers calc. BC
(1000:980)x(480,9:517,7)x0,458= 0,433 (BC corrected for pressure (in hPa, works same way for inches Hg if I understand the theory correct) and temperature (simple temperature measurement converted to Rankine).

[ 07-23-2004: Message edited by: sierra22 ]

[ 07-23-2004: Message edited by: sierra22 ]

[ 07-23-2004: Message edited by: sierra22 ]

5. ### MysticplayerWriters Guild

Messages:
1,459
Joined:
Jul 27, 2001
This really isn't a cop out but the best way to account for all variables on the day you point your muzzle at your target game.

At long range, there are so many variables that simply can't be calculated out if you want to be certain that you can hit your target.

The safest is take a spotter shot or two a few hundred yds from your game. Adjust as necessary. Get on the target.

It only takes a few more seconds, confirms everything, and doesn't disturb the game.

At ranges 700 to 1700yds, this helps me to be sure.

Jerry

6. ### kidcoltoutlawWell-Known Member

Messages:
339
Joined:
Dec 18, 2001
how about perry systems on a palm.it works.

7. ### 700Well-Known Member

Messages:
138
Joined:
Jun 30, 2002
Sierra22

Send me a message email by clicking on the first envelope symbol above.

I will prob. be able to help you out with charts that you can print off and bring to the range with you.

I will need to get basic ballistic data from you.

Kindest regards

700

Messages:
138
Joined:
Jun 30, 2002
9. ### JBMWell-Known Member

Messages:
113
Joined:
Jan 12, 2004
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>PROBLEM: Correcting trajectory for measured barometric pressure and temperature to score first round hits without sighters.
Trajectories are to my understanding calculated according to a standard atmosphere. Change this and the bullet will behave differently and the calculated BC will change.
<HR></BLOCKQUOTE>

Yes it will. Also the speed of sound will change as a function of temperature and this will also affect the calculations (but not as much).

Don't forget the humidity correction (it is pretty small though).

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>
...then I can calculate the deviations (not correctly, but better than not doing it) by working out a new dropchart for the new BC.
<HR></BLOCKQUOTE>

It's not that bad -- but if you're calculating a new drop chart, you might as well just calculate one with new atmospheric conditions. If you meant that you're finding correction factors for the old chart, then keep in mind that the mach number is calculated using the conditions of the chart not the same conditions of the new BC.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>
I WANT: To find correction factors where I can correct directly from a trajectory chart without going the long way around and making a new one.<HR></BLOCKQUOTE>

Don't we all! Actually this has been my gripe with ballistics tables books that offer a bunch of tables. They're aren't much good when the conditions change. I'm working on some software to calculate the tables (just about done) and I am also developing some methods to correct the tables for different BCs, zero ranges and scope height. If I can find a publisher, I'll get it published. I don't really want to go the self publishing route. I hate hijacking threads, but is this something that might be useful (i.e. would you buy it)? I'd include the software so that you could make custom tables.